This invention is generally directed to coating processes, and more specifically to the coating of various components in an imaging and printing apparatus. More specifically, the present invention is directed to the coating of heating elements in ink jet, and particularly thermal ink jet apparatus, which coatings can, for example, be abrasion, cavitation, and chemically resistant, electrically insulating, and function as a thermal conductor. In one embodiment of the present invention, the coating selected is comprised of diamond, or polycrystalline diamond. More specifically, the present invention in a specific embodiment relates to the coating of silicon heaters in thermal ink jet printing systems with polycrystalline diamond thereby enabling a passivation layer. Passivation refers in embodiments to protection against mechanical and chemical attack from external agents such as the inks selected. The polycrystalline diamond having a hardness comparable to single crystal diamond, the hardest known material, avoids or minimizes undesirable abrasion and cavitation caused by bubbles formed in the inks, thereby extending the lifetime of the components coated. The polycrystalline diamond, which is unaffected by either acids or bases as well as organic solvents, also avoids or minimizes undesirable chemical interactions between the inks and the heaters. Although no described to be limited by theory it is believed that since undoped polycrystalline diamond is an electrical insulator, it prevents the electrical current used to power the heaters from being transferred to the inks thus preventing electrolysis of the inks and undesirable reactions between the inks and heaters which are activated by the electric power.
In addition, diamond is an efficient conductor of heat from the heating elements or thermoelectrical transducers to the in the ink for a comparable thickness of tantalum or amorphous carbon needed to provide passivation, thus reducing the power requirements needed for the operation of the device. In Handbook of Chemistry and Physics, CRC Press, Boco Raton, Fla., R. C. Weast, Editor, 1983, page E-11, the thermal conductivity at 298.2 K is given for the following materials:
______________________________________ Copper 4.01 Wcm.sup.-1 K.sup.-1 Tantalum 0.575 Amorphous-Carbon 0.0159 Diamond, Type I 9.90 Diamond, Type IIa 23.2 Diamond, Type IIb 13.6 ______________________________________
Diamond of any type is considered superior to tantalum or amorphous-carbon, "diamond-like carbon", in terms of thermal conductivity. In a paper by A. Ono, T. Baba, H. Funamoto and A. Nishikawa, Japanese Journal of Applied Physics, 25 (1986) L808, there is indicated that the thermal conductivity of CVD polycrystalline diamond thin films is strongly depended on the amount of diamond character in the films and thus on the Raman peak at 1,330 cm.sup.-1. The thermal conductivity drops by a factor of 100 when the films have a high amorphous content as shown by the increase in the Raman peak at 1,500 cm.sup.-1 and a decrease in the peak at 1,330 cm.sup.-1. Polycrystalline diamond thin films are shown to have a thermal conductivity close to that of single crystal diamond, Type IIb, which ia a factor of 2 higher than copper at 100.degree. to 130.degree. C., whereas amorphous carbon of "diamond-like carbon" has a thermally conductivity less than diamond and even less than copper.
Printing methods utilizing ink jet technologies as well known and ink jet printers are commercially available from, for example, Xerox Corporation as the ink jet printer 4020.TM.. Also, there is illustrated, for example, in U.S. Pat. Nos. 4,335,369, 4,392,907 and 4,794,410, the disclosures of which are totally incorporated herein by reference, a particular ink jet printing technology which is based on thermal rather than electrostatic ink acceleration methods ink jet printing methods generally involve the physical separation of a predetermined and metered quantity of ink, which could be a dye based or a pigmented fluid material from an orifice. In these processes, ink jet heaters with, for example, coatings of silicon nitride and tantalum are selected. The silicon nitride layer is selected to provide electrical insulation between the heaters and the ink. The tantalum layer transfers the heat from the silicon heaters to the ink and acts as a compliant mechanical buffer to minimize cavitation damage. Since diamond is an electrical insulator, extremely hard and an excellent conductor of heat, a single layer of diamond can replace the two layers of silicon nitride and tantalum thereby decreasing the number of processing steps required to make the thermal ink jet device. When the tantalum layer is made to be 1 micron thick, it resists cavitation sufficiently well for the desirable operation of the device. However, since tantalum is not a particularly good conductor of heat, the power requirements for the heating elements become undesirably high. If the thickness of the tantalum layer is decreased to 0.5 micron, the power requirements for the heaters are reasonable, but in time the tantalum layer degrades due to cavitation damage from the bubbles in the inks. Diamond being both extremely hard and an excellent conductor of heat eliminates the above problems.
The aforementioned and other disadvantages are avoided with the processes of the present invention wherein polycrystalline diamond is selected.
Other thermal ink jet printing processes and apparatuses wherein the coatings of the present invention may be selected are illustrated in U.S. Pat. Nos. 4,639,748; 4,864,329 and U.S. Pat. Re. 32,572, the disclosures of each of the aforementioned patents being totally incorporated herein by reference.
In U.S. Pat. No. 4,925,701, the disclosure of which is totally incorporated herein by reference, there are illustrated processes for the preparation of polycrystalline diamond films, which films can be selected for the processes of the present invention in embodiments thereof. More specifically, this patent discloses a process for the preparation of continuous polycrystalline diamond films, which comprises applying to a substrate diamond powder in an amount of from about one particle per ten square microns to about 10 particles per square micron with an average particle diameter of from about 0.1 to about 0.4 micron, heating the resulting powdered substrate subsequent to incorporation in a processing apparatus; introducing a mixture of gases into the apparatus, which gases provide a supply of carbon and hydrogen; and decomposing the gas mixture.
In a patentability search report, the following United States patents are recited: U.S. Pat. No. 4,847,639, which discloses an ink jet recording head with liquid passages and electrothermal transducers for generating heat containing a heat generating resistance layer of an amorphous material containing halogen and hydrogen atoms in a matrix of carbon atoms; and U.S. Pat. Nos. 4,740,263; 4,830,702; 4,842,945; 4,844,785; 4,859,493; 4,869,923 and 4,919,974 all relating to diamond like thin films. The aforementioned search report mentions as collateral interest U.S. Pat. Nos. 4,681,653 and 4,747,922; and the Journal of Applied Physics, Volume 42, 1971, pp. 2953 to 2958, which relates to diamond like thin films.
Illustrated in copending patent application U.S. Ser. No. 624,031 the disclosure of which is totally incorporated herein by reference, is a selective seeding process of diamond films and wherein thermal ink jet heater protection pads are applied by this process. More specifically, there is disclosed in this application a method of forming a patterned, polycrystalline diamond film on a substrate, comprising applying to a substrate a photoresist layer; applying to said photoresist layer a diamond powder layer, said diamond power layer including diamond particles; exposing said photoresis layer and said diamond powder layer to electromagnetic radiation through a mask; developing said photoresis layer sufficiently to form a developed photoresis layer; heating the substrate, developer photoresis layer, and diamond powder layer in a processing apparatus, said heating step casing said developed photoresist layer to carbonize; introducing a mixture of gases into said apparatus, said mixture of gases including carbon-containing and hydrogen-containing gases; and decomposing said mixture of gases in said apparatus, whereby hydrogen in said hydrogen-containing gases remove said carbonized photoresist layer, and whereby carbon in said carbon-containing gases combines with said diamond particles forming diamond structures on said substrate.
Illustrated in U.S. Pat. No. 5,073,785, the disclosure of which is totally incorporated herein by reference, is a process for minimizing or avoiding drop deflection in ink jet devices which comprises coating the ink jet head components with amorphous carbon.